Brief Description Of The Prior Art
Center pivot irrigation systems used for irrigating large cultivated areas in the Great Plains of the United States frequently include large pipes by which water for irrigation is piped extensive distances from a central source, with such pipes supported on large wheels which may be many feet in diameter. The site which is being irrigated is changed from time-to-time by pulling the system to a new location upon the supporting wheels as the wheels are rotated to accommodate the movement when the system is pulled to the new location by a tractor. The wheels are usually provided in pairs at spaced locations along the irrigation pipe called towers.
The weight supported by each wheel is great with the result that frequently there is a marked propensity for the wheel to sink deeply into the earth, and to make ruts of considerable depth as it is undergoing movement from on location to the other. This presents an obstacle to the movement of personnel and vehicles across the field so rutted. Accordingly, some effort has been made to construct irrigation wheels of the type described which better support the weight imposed upon them, and resist being forced to a significant depth in the earth by such weight.
One such irrigation wheel is sold under the trade name "E-Z Trak" by Caldwell Manufacturing Company of Kerney, Nebr. The "E-Z Trak" irrigation wheel includes a pair of outer rim plates which contact the ground and support the wheel thereon. These rims are spaced from each other, and are angled so that they actually each constitute a frustum of a cone. Stated differently, the inner peripheral edge of each annular rim in the "E-Z Trak" wheel is located more closely to the axis of rotation of the wheel than is the outer edge of each of the rims. Stated yet another way, each rim plate slopes radially inwardly from its outer edge to the central plane of the wheel and to the two inner edges of the two rim plates which face each other across an intervening gap or space which separates the two rims. The rim plates as thus configured and located are retained in this position by radial spoke plates which are located in circumferentially spaced pairs extending from a central hub outwardly to the rims.
The "E-Z Trak" irrigation wheel has achieved some improvement over the older type of irrigation wheel, which simply had a steel banded single rim at the outer periphery thereof and which allowed the wheel to sink deeply into the earth and cause the rutting hereinbefore described. The "E-Z Trak" wheel accomplished some improvement in terms of resistance to rutting and sinking into damp earth. This occurred because the spaced, inwardly inclined rim plates tended to force or push the earth between the rims inwardly toward the central plane of the wheel, and to thus compact the earth in a way which supported the wheel better and prevented it from sinking deeply into the earth as a single rut was formed.
The "E-Z Trak" irrigation wheel, which is now patented as U.S. Pat. No. 5,046,785, though constituting an improvement over the simple single rim wheels of the past, has allowed undesirably large quantities of mud and earth to be impacted between the rim and the central hub of the wheel due to the flat, plate-like construction of the spokes, and the relatively close spacing of some of the spokes to each other. Moreover, the inclination of the rim plates by which they slope radially inwardly from the outer edge to the inner edge has been such that the earth has been forced inwardly into the center of the wheel, and from that location into the space between the rim plates and the central hub in greater quantities than would be desirable. The dirt and mud clogs the central portion of the irrigation wheel and causes it to become very bulky and heavy as a result of the accumulation of mud at this location. Such increase in weight aggravates he rutting problem.
An improved irrigation wheel which can be operated for longer periods of time is described in our co-pending U.S. patent application Ser. No. 07/649,971 filed on Feb. 14, 1991. That irrigation wheel includes a central hub plate which defines a central opening. The central hub plate is concentrically located with respect to the axis of rotation of the irrigation wheel, and is adapted to be drivingly connected to a gear box located at one of the irrigation system towers.
The central hub plate is concentrically surrounded by a pair of rim plates which are spaced from each other and from the central hub plate, and extend substantially parallel to each other. Each of the rim plates is in the shape of a frustum of a cone, thus having one of its peripheral edges (referred to the inner edge) located a closer radial distance to the axis of rotation of the wheel than its outer edge is located. Stated differently, each of the two rim plates slope radially inwardly slightly toward the central plane, with this inward slope being from the outer edge of the paired rim plates in the direction of the space which separates the rim plates. This configuration of the rim plates tends to compact the mud across which the wheel is moving, and to thus provide greater support to the irrigation wheel, thereby reducing the tendency of the wheel to sink downwardly in the mud and cause deep rutting.
The two rim plates are connected to the central hub plate by a plurality of bars or rods which form spokes maintaining the concentric relationship between the rim plates and the hub plate. The radially inner ends of these spoke rods or bars are secured to the hub plate, and the outer ends of the several spoke rods ar welded to transversely extending wheel lugs which are elongated plates extended across, and welded to the pair of rim plates. The wheel lugs occupy the axial space between the rim plates.